Well, I've done another square using 6mm needles (on the basis that if the required tension is between the two I'll go down to 5.5mm), washed both squares, and am waiting for them to dry.
As I want to make the jacket a little bigger than the biggest size on the pattern (why do books of patterns so often give such limited sizes??) I may decide to use the 6mm needles even if the tension is looser than required - the knitting has come out quite nicely with both squares. It would be easier than trying to factor in extra pattern repeats.